This invention relates to an infrared heater, and more particularly to an infrared heater which emits far-infrared rays in a wave range of 3 .mu.m or longer for use in effecting reflowing of solder, curing of resins, drying of food, heating of wood and wet coatings, warming for medical treatment, and the like.
Recently, there is a general trend for electronic equipment to be made increasingly compact and light weight. Accordingly, printed circuit boards having a large number of electronic parts mounted in a limited area (hereunder referred to as "high-density mounted boards" or "high-density mounted printed circuit boards") are widely used. In the manufacture of high-density mounted boards, it is necessary to supply heat to a narrow area between electronic parts on the high-density mounted board in order to reflow a paste solder or to cure an adhesive resin when the electronic parts are connected to the circuit board using a paste solder or a resinous bonding agent. As an industrial heating apparatus for these purposes, a reflowing furnace is used in which infrared heaters are placed on the top and bottom walls of a tunnel-type heating zone. The infrared heater used in the reflowing furnace comprises a sheath heater, or a mere heater supported by a steel plate. The radiation wavelength is not longer than 3 .mu.m.
Since infrared rays travel in straight lines, the electronic parts on the high-density mounted board prevent the infrared rays from directly reaching the area where solder or adhesive resin was previously placed. Thus, sometimes the reflowing of a paste solder or the curing of a bonding agent is not sufficient to effect bonding of the electronic parts to the circuit board.
In addition, infrared rays having a maximum wavelength of 3 .mu.m, which are produced by conventional infrared heaters, e.g., infrared lamps for use in a reflowing furnace, are not well absorbed by white objects, especially by a metal such as solder. Therefore, in order to thoroughly melt the solder in a reflowing furnace, it is necessary to increase the amount of heat to be generated by a heating element by increasing the electric current density of the sheath heater. Unfortunately, since infrared rays of a wavelength of 3 .mu.m or shorter are easily absorbed by black objects, and electronic parts usually have a black exterior, the electronic parts are preferentially heated. Thus, when the temperature is increased in order to efficiently heat the paste solder or bonding agent, e.g., by increasing the current denisty as described above, the electronic parts are inevitably further heated, resulting in thermal damage which can produce the malfunction of the parts.